This document provides information about measuring solar radiation. It defines key terms like beam, diffuse, and total radiation. It also describes different types of instruments used to measure solar radiation, including pyrheliometers and pyranometers. Pyrheliometers are used to measure direct beam radiation from the sun, and come in designs like the Angstrom and silver disk models. Pyranometers measure global radiation from the entire sky, and common types are thermoelectric models. Sources of measurement errors for these instruments are also outlined.

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REPORT
NOVEMBER 10
ALEXANDRIA UNIVERSITY
Prepared by: Sammy Jamar Chemengich
SOLAR RADIATION
MEASUREMENTS

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SOLAR RADIATION MEASUREMENTS
1.0 Definition of terms
Everything in nature emits electromagnetic energy, and solar radiation is energy emitted by the sun.
The energy of extraterrestrial solar radiation is distributed over a wide continuous spectrum ranging
from ultraviolet to infrared rays.
The fraction of the energy flux emitted by the sun and intercepted by the earth is characterized by
the solar constant. The solar constant is defined as a flux density measuring mean solar
electromagnetic radiation (solar irradiance) per unit area. It is most precisely measured by satellites
outside the earth atmosphere. The solar constant is currently estimated at 1367 W/m2 [cited from
Stine and Harrigan, 1986]. This number actually varies by 3% because the orbit of the earth is
elliptical, and the distance from the sun varies over the course of the year.
The radiation that comes directly from the sun is defined as beam radiation. The scattered and
reflected radiation that is sent to the earth surface from all directions (reflected from other bodies,
molecules, particles, droplets, etc.) is defined as diffuse radiation. The sum of the beam and diffuse
components is defined as total (or global) radiation.
Wavelength (µm)
FIG 1.1 Spectrum distribution of solar radiation
A) Extraterrestrial solar radiation
(B) Direct solar radiation on the
earth’s surface
(C) Blackbody radiation at 5,900 K
(Shaded areas indicate absorption by
the atmosphere)

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Short-wave radiation, in the wavelength range from 0.3 to 3 μm, comes directly from the sun. It
includes both beam and diffuse components.
Long-wave radiation, with wavelength 3 μm or longer, originates from the sources at near-ambient
temperatures - atmosphere, earth surface, light collectors, other bodies.
The solar radiation reaching the earth is highly variable and depends on the state of the atmosphere
at a specific location. Two atmospheric processes can significantly affect the incident radiation:
scattering and absorption.
Scattering is caused by interaction of the radiation with molecules, water, and dust particles in the
air. How much light is scattered depends on the number of particles in the atmosphere, particle size,
and the total air mass the radiation comes through.
Absorption occurs upon interaction of the radiation with certain molecules, such as ozone
(absorption of short-wave radiation - ultraviolet), water vapor, and carbon dioxide (absorption of
long-wave radiation - infrared).
Due to these processes, out of the whole spectrum of solar radiation, only a small portion reaches the
earth surface. Thus, most of x-rays and other short-wave radiation is absorbed by atmospheric
components in the ionosphere, ultraviolet is absorbed by ozone, and not-so abundant long-wave
radiation is absorbed by CO2. As a result, the main wavelength range to be considered for solar
applications is from 0.29 to 2.5 μm [Duffie and Beckman, 2013].
Figure 1.2. Different types of radiation at the earth surface: orange - short wave; blue -
long wave.

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2.Solar measuring instruments
1. Pyrheliometer
A pyrheliometer is used to measure direct solar radiation from the sun. To measure direct solar
radiation correctly, its receiving surface must be arranged to be normal to the solar direction. For this
reason, the instrument is usually mounted on a sun-tracking device called an equatorial mount.
Sunlight enters the instrument through a window and is directed onto a thermopile which converts
heat to an electrical signal that can be recorded. The signal voltage is converted via a formula to
measure watts per square metre.
a) Angstrom electrical compensation pyrheliometer
Fig 1.3 Angstrom electrical compensation pyrheliometer
This pyrheliometer has a rectangular aperture, two manganin-strip sensors (20.0 mm × 2.0 mm ×0.02
mm) and several diaphragms to let only direct sunlight reach the sensor.
The sensor surface is painted optical black and has uniform absorption characteristics for short-wave
radiation. A copper-constantan thermocouple is attached to the rear of each sensor strip, and the
thermocouple is connected to a galvanometer. The sensor strips also work as electric resistors and
generate heat when a current flow across them.
When solar irradiance is measured with this type of pyrheliometer, the small shutter on the front face
of the cylinder shields one sensor strip from sunlight, allowing it to reach only the other sensor. A
temperature difference is therefore produced between the two sensor strips because one absorbs
solar radiation and the other does not, and a thermoelectromotive force proportional to this
difference induces current flow through the galvanometer. Then, a current is supplied to the cooler
sensor strip (the one shaded from solar radiation) until the pointer in the galvanometer indicates
zero, at which point the temperature raised by solar radiation is compensated by Joule heat. A value
(a) Structure
(b) Circuit
A: Aperture B: Battery C: Sensor surface D:
Cylinder
P: Switch R: Variable resistor S: Shutter
T: Thermocouple G: Galvanometer

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for direct solar irradiance is obtained by converting the compensated current at this time. If S is the
intensity of direct solar irradiance and i is the current, then
S = Ki2,
where K is a constant intrinsic to the instrument and is determined from the size and electric resistance of
the sensor strips and the absorption coefficient of their surfaces. The value of K is usually determined through
comparison with an upper-class standard pyrheliometer.
b) Silver-disk pyrheliometer
Fig 1.4 Silver disk pyrheliometer
The sensing element is a silver disk measuring 28 mm in diameter with a thickness of 7 mm that is painted
black on its radiation-receiving side. It has a hole from the periphery toward the center to allow insertion of
the bulb of a high-precision mercury-in-glass thermometer. To maintain good thermal contact between the
disk and the bulb, the hole is filled with a small amount of mercury. It is enclosed outside by a heat-insulating
wooden container. The stem of the thermometer is bent in a right angle outside the wooden container and
supported in a metallic protective tube. A cylinder with diaphragms inside is fitted in the wooden container to
let direct solar radiation fall onto the silver disk. There is a metallic-plate shutter at the top end of the cylinder
to block or allow the passage of solar radiation to the disk.
During the measurement phase, the disk is heated by solar radiation and its temperature rises. The intensity of
this radiation is ascertained by measuring the temperature change of the disk between the measurement
phase and the shading phase with the mercury-in-glass thermometer
a: Black-painted silver disk
b: Mercury-in-glass thermometer
c: Wooden container
d: Copper case
e: Cylinder
f: Diaphragm
g: Shutter

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This instrument uses a thermopile at its sensor, and continuously delivers a thermoelectromotive force in
proportion to the direct solar irradiance. While Angstrom electrical compensation pyrheliometers and silver-
disk pyrheliometer have a structure that allows the outer air to come into direct contact with the sensor
portion, this type has transparent optical glass in the aperture to make it suitable for use in all weather
conditions. It is mounted on a sun-tracking device to enable outdoor installation for automatic operation.
In this pyrheliometer, a temperature difference is produced between the sensor surface (called the hot
junction) and the reference temperature point, i.e., the metallic block of the inner cylinder (called the cold
junction). As the temperature difference is proportional to the intensity of the radiation absorbed, the level
of solar radiation can be derived by measuring the thermoelectromotive force from the thermopile. Since
this type of pyrheliometer is a relative instrument, calibration should be performed to determine the
instrumental factor through comparison with a standard instrument. As the thermoelectromotive force
output depends on the unit’s temperature, the temperature inside the cylinder should be monitored to enable
correction.

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2.0 Pyranometers
A pyranometer is a type of actinometer used for measuring solar irradiance on a planar surface and it
is designed to measure the solar radiation flux density (W/m2) from the hemisphere above within a
wavelength range 0.3 μm to 3 μm.
Its sensor has a horizontal radiation-sensing surface that absorbs solar radiation energy from the
whole sky (i.e. a solid angle of 2π sr) and transforms this energy into heat. Global solar radiation can
be ascertained by measuring this heat energy. Most pyranometers in general use are now the
thermopile type, although bimetallic pyranometers are occasionally found.
a) Thermoelectric pyranometers
The instrument’s radiation-sensing element has basically the same structure as that of a
thermoelectric pyrheliometer. Another similarity is that the temperature difference derived between
the radiation-sensing element (the hot junction) and the reflecting surface (the cold junction) that
serves as a temperature reference point is expressed by a thermopile as a thermoelectromotive
force. In the case of a pyranometer, methods of ascertaining the temperature difference are as
follows:
i. Several pairs of thermocouples are connected in series to make a thermopile that
detects the temperature difference between the black and white radiation-sensing
surfaces
ii. The temperature difference between two black radiation-sensing surfaces with
differing areas is detected by a thermopile.
iii. The temperature difference between a radiation-sensing surface painted solid black
and a metallic block with high heat capacity is detected by a thermopile
The radiation-sensing element (in the upper right of the figure) consists of two pairs of bimetals, one
painted black and the other painted white, placed in opposite directions (face up and face down) and
attached to a common metal plate at one end. At the other end, the white bimetallic strips are fixed
to the frame of the pyranograph, and the black ones are connected to the recorder section via a
transmission shaft. The deflection of the free edge of the black strips is transmitted to the recording
pen through a magnifying system. When the air temperature changes, the black and white strips
attached to the common plate at one end both bend by the same amount but in opposite directions.
As a result, only the temperature difference attributed to solar radiation is transmitted to the
recording pen.
Thermoelectric pyranometers and bimetallic pyranographs are both hermetically sealed in a glass
dome to protect the sensor portion from wind and rain and prevent the sensor surface temperature
from being disturbed by wind. A desiccant is placed in the dome to prevent condensation from
forming on the inner surface. The glass allows the passage of solar radiation in wavelengths from
about 0.3 to 3.0 µm – a range that covers most of the sun's radiation energy. Some models are

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equipped with a fan to prevent dust or frost, which greatly affect the amount of light received, from
collecting on the dome’s outer surface. It is necessary to check and clean the glass surface at regular
intervals to ensure that the dome wall constantly allows the passage of solar radiation.
Source of errors
i. Wavelength Characteristics (for pyrheliometers and pyranometers)
The absorption coefficient of the radiation sensor surface and the transmission coefficient of the
glass cover or glass dome should be constant for all wavelengths of solar radiation. In reality,
however, these coefficients vary with wavelength. Since this wavelength characteristic differs slightly
from one instrument to another, observation errors occur when the energy distribution of solar
radiation against wavelength varies with the sun’s elevation or atmospheric conditions.
ii. Temperature Characteristics (for pyrheliometers and pyranometers)
As the thermoelectromotive force of a thermopile is nonlinear and the heat conductivity inside an
instrument depends on temperature; the sensitivity of these instruments varies and an error occurs
when the ambient temperature and the temperature of the instrument change.
iii. Characteristics against Elevation and Azimuth (for pyranometers)
The output of the ideal pyranometer decreases with lower sun elevation angles in proportion to cosZ
(Z: zenith angle). In reality, however, output varies with the sun’s elevation or azimuth due to the
uneven absorption coefficient and with the shape of the radiation sensor surface. The characteristic
may also deviate and errors may occur because of the uneven thickness, curvature or material of the
glass cover. Usually, sensitivity rapidly decreases at an elevation angle of around 20 degrees or
lower.
iv. Field of View (for pyrheliometers)
The field of view of a pyrheliometer is somewhat larger than the viewing angle of the sun. If the
field of view differs, the extent of influence from diffuse sky radiation near the sun also differs.
Pyrheliometers with different fields of view may make different observations depending on the
turbidity of the atmosphere. (WMO recommends a total opening angle of five degrees.)
Sitting and exposure
Pyranometers in particular should be installed at a site where the instrument is not influenced by
intense reflected light from the wall surfaces of buildings.

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REFERENCES
1.World Bank. 2017. Global Solar Atlas. https://globalsolaratlas.info
2. Wang, Y.-M.; Lean, J. L.; Sheeley, N. R. (2005). "Modeling the Sun's magnetic field and irradiance since
1713
3. Steinhilber, F.; Beer, J.; Fröhlich, C. (2009). "Total solar irradiance during the Holocene".
4. Lean, J. (14 April 1989). "Contribution of Ultraviolet Irradiance Variations to Changes in the Sun's Total
Irradiance". Science
5. ISO 9060:1990 Classification of Pyranometers
6. World Radiometric Reference